acos, acosf, acosl

Defined in header `<math.h>`
`float acosf( float arg );`	(1)	(since C99)
`double acos( double arg );`	(2)
`long double acosl( long double arg );`	(3)	(since C99)
`_Decimal32 acosd32( _Decimal32 arg );`	(4)	(since C23)
`_Decimal64 acosd64( _Decimal64 arg );`	(5)	(since C23)
`_Decimal128 acosd128( _Decimal128 arg );`	(6)	(since C23)
Defined in header `<tgmath.h>`
`#define acos( arg )`	(7)	(since C99)

1-6) Computes the principal value of the arc cosine of arg.

7) Type-generic macro: If the argument has type long double, (3) (acosl) is called. Otherwise, if the argument has integer type or the type double, (2) (acos) is called. Otherwise, (1) (acosf) is called. If the argument is complex, then the macro invokes the corresponding complex function (cacosf, cacos, cacosl).

The functions (4-6) are declared if and only if the implementation predefines __STDC_IEC_60559_DFP__ (i.e. the implementation supports decimal floating-point numbers).

(since C23)

Parameters

arg

floating-point value

Return value

If no errors occur, the arc cosine of arg (arccos(arg)) in the range [0 ; π], is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

Domain error occurs if arg is outside the range [-1.0; 1.0].

If the implementation supports IEEE floating-point arithmetic (IEC 60559):

If the argument is +1, the value +0 is returned;
If |arg| > 1, a domain error occurs and NaN is returned;
if the argument is NaN, NaN is returned.

Example

#include <errno.h>
#include <fenv.h>
#include <math.h>
#include <stdio.h>
#include <string.h>
 
#ifndef __GNUC__
#pragma STDC FENV_ACCESS ON
#endif
 
int main(void)
{
    printf("acos(-1) = %f\n", acos(-1));
    printf("acos(0.0) = %f 2*acos(0.0) = %f\n", acos(0), 2 * acos(0));
    printf("acos(0.5) = %f 3*acos(0.5) = %f\n", acos(0.5), 3 * acos(0.5));
    printf("acos(1) = %f\n", acos(1));
 
    // error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("acos(1.1) = %f\n", acos(1.1));
    if (errno == EDOM)
        perror("    errno == EDOM");
    if (fetestexcept(FE_INVALID))
        puts("    FE_INVALID raised");
}

Possible output:

acos(-1) = 3.141593
acos(0.0) = 1.570796 2*acos(0.0) = 3.141593
acos(0.5) = 1.047198 3*acos(0.5) = 3.141593
acos(1) = 0.000000
acos(1.1) = nan
    errno == EDOM: Numerical argument out of domain
    FE_INVALID raised

References

C23 standard (ISO/IEC 9899:2023):

7.12.4.1 The acos functions (p: TBD)
7.25 Type-generic math <tgmath.h> (p: TBD)
F.10.1.1 The acos functions (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.12.4.1 The acos functions (p: 173)
7.25 Type-generic math <tgmath.h> (p: 272-273)
F.10.1.1 The acos functions (p: 378)

C11 standard (ISO/IEC 9899:2011):

7.12.4.1 The acos functions (p: 238)
7.25 Type-generic math <tgmath.h> (p: 373-375)
F.10.1.1 The acos functions (p: 518)

C99 standard (ISO/IEC 9899:1999):

7.12.4.1 The acos functions (p: 218)
7.22 Type-generic math <tgmath.h> (p: 335-337)
F.9.1.1 The acos functions (p: 455)

C89/C90 standard (ISO/IEC 9899:1990):

4.5.2.1 The acos function

asinasinfasinl (C99)(C99)	computes arc sine (\({\small\arcsin{x} }\)arcsin(x)) (function)
atanatanfatanl (C99)(C99)	computes arc tangent (\({\small\arctan{x} }\)arctan(x)) (function)
atan2atan2fatan2l (C99)(C99)	computes arc tangent, using signs to determine quadrants (function)
coscosfcosl (C99)(C99)	computes cosine (\({\small\cos{x} }\)cos(x)) (function)
cacoscacosfcacosl (C99)(C99)(C99)	computes the complex arc cosine (function)
C++ documentation for `acos`

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